There is increasing concern over the hazards posed by the rising levels of heavy metals in the world's water supplies. As used herein, the term "heavy metals" refers to both the cationic and anionic forms of non-ferrous metals and metalloids (e.g., arsenic) which have an atomic number greater than that of calcium. Most heavy metals are deemed toxic to some degree to all life-forms. In humans, toxic heavy metal poisoning can lead to severe nervous system disorders and, in extreme cases, death. Even trace amounts of heavy metals are potentially dangerous, because they do not decompose over time and often accumulate within an organism throughout its lifetime. This accumulation effect may be accentuated in succeeding species along the food chain.
As a consequence of this problem, "industry" is being forced to use treatment processes capable of virtually eliminating heavy metals from aqueous wastes. Often, however, this requirement is very difficult to fulfill. The metal finishing industry, for example, employs a variety of processes which, typically, generate large volumes of aqueous waste material. Many of these wastes contain unacceptable concentrations of heavy metals such as zinc, nickel, copper, chromium, lead, cadmium, iron, tin, gold, and silver. Since over a billion gallons of such wastes are generated daily by the 8000 or so metals finishing plants operating in the United States, it is obvious that a considerable amount of toxic waste solution is formed.
In response to this, numerous heavy metal removal methods including evaporation/crystallization, absorption, dialysis, electrodialysis, reverse osmosis and ion exchange have been proposed for the metals finishing and plating industries, with varying degrees of success. Another method is to sequester and remove toxic heavy metals in a flocculated coagulant comprised of one or more hydrated oxides of iron. A common method for removing these metals is by alkali precipitation, forming a semi-solid "sludge" of insoluble metal hydroxides, and disposing of this material by depositing it in a certified "toxic waste" landfill.
The heavy metal containing "sludge" material is typically toxic, but not primarily composed of heavy metals. Large amount of water or carrier materials are included with the heavy metals, creating additional materials for toxic disposal. However, the number of toxic waste landfills is relatively low and the price for disposing these wastes is increasing. For example, fees, transportation costs, and other expenses for such disposal can easily reach as much as $500/ton of sludge. This is besides the treatment and removal method operating costs, including chemicals, which can be significant. It would be highly desirable if the amount of sludge could be reduced and the removal method operating costs could be diminished.